This invention relates to solar energy and in particular to a wing design for a solar powered aircraft.
A solar powered aircraft is one which derives energy from the sun's rays by means of photovoltaic solar cells. This energy may be transformed for use in an electric motor to power the aircraft. Excess energy is stored either as potential energy in the form of increased altitude of the aircraft, or as energy stored onboard in batteries or in some other energy storage system. The stored energy is used to keep the vehicle aloft during the night, so that flight time is not limited by fuel supply as it is on a conventional aircraft. The vehicle must fly at an altitude high enough to be above the clouds, and to avoid winds with velocities much higher than the airspeed of the vehicle. Because of its long endurance and limited weight-carrying ability, this type of vehicle is normally considered to be a pilotless aircraft.
Prior art discloses solar powered aircraft, of conventional configuration with solar cells on top of the wing or fuselage. U.S. Pat. No. 3,089,670 describes an aircraft wherein the wing surfaces are provided with solar cells. A rotary type (flying saucer) aircraft with solar cells located on rotor blades is disclosed in U.S. Pat. No. 3,744,865. For both of these aircraft, the energy available is maximum when the sun is directly overhead, and decreases approximately as the cosine of the angle from the normal to the plane of the solar cells. Operation of such solar powered aircraft is therefore limited by the position of the sun.
Desired capabilities of a solar powered aircraft include the ability to remain airborne for an indefinite length of time, and to maintain a station over a desired location. To accomplish these ends, the ability to recover the maximum possible solar energy must be maintained throughout daylight hours. One method for maintaining the output of solar cells at various sun angles involves placing the solar cells in vertical as well as horizontal planes. To perform station keeping, the aircraft must fly a racetrack pattern, and solar cells are required on both sides of the wing and fuselage. This increases by a factor of three the area of solar cells required to yield a power output throughout the daylight hours equal to that produced with cells only on the top of the wing and the sun directly overhead. The obvious weight disadvantage makes this option impractical.
Banking the aircraft so that the normal to the wing always lies along the sun line would eliminate the weight disadvantage just described. A conventional aircraft, however, cannot maintain straight flight at a large bank angle.
Also suggested has been placement of solar cells on a tilting panel within a transparent fuselage structure. Such placement would require inclusion of a cooling system for inner cells; the associated weight and aerodynamic drag penalties again limit the feasibility of the concept.
U.S. Pat. No. 3,025,027 describes a vertical airfoil which produces a lift and may support an aircraft in flight. One such vertical airfoil is used with two horizontal airfoils.
An object of the present invention is to provide a solar powered aircraft which can maintain maximum exposure of its solar cells to the sun.
An object of the present invention is to allow full power output of solar cells at any location on earth and at any time during daylight hours.
Another object of the present invention is to provide such an aircraft, which aircraft may be banked to align its solar cells normal to the sun line.
Another object of the present invention is to provide such an aircraft, which aircraft may be guided to fly a station-keeping pattern.
Another object of the present invention is to provide such an aircraft, wherein solar cells are located on only one airfoil surface.